In some existing approaches of wafer preparation, a positive photoresist layer is used without a wafer edge exposure (WEE) process. The photoresist layer, however, is easily peeled off the wafer. In a process using the positive photoresist layer with the WEE process, various problems exist at the wafer edge. For example, at a resist protective oxide (RPO) layer, a film thickness is reduced because the film including oxide in the RPO layer and/or an inter-metal dielectric (IMD) layer is removed at the wafer edge. Metal such as silicide or copper is high at the WEE area. At the metal layers, copper from a copper deposition step exists at the wafer edge, and results in step-height or wafer non-uniformity. During wafer trimming of a back side illumination (BSI) process, the WEE process also causes non-uniformity at the wafer edge. To prevent wafer cracks, wafer trimming is used to remove the areas of non-uniformity. Metal powder from wafer trimming also results in a high metal content. The metal tends to migrate to other areas of the wafer and contaminates functional dies. Metal at the wafer edge also affects a dark current of image-sensors and thus affects performance of the image-sensors.
Like reference symbols in the various drawings indicate like elements.